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Spotlight on Optics

Highlighted Articles from OSA Journals

June 2010

Spotlight Summary by Ertugrul Cubukcu

Polymeric light delivery via a C-shaped metallic aperture

Why do you need a laser in your future hard drive? –The magnetic hard drive is still the major player for high-density data storage. The technology used in today’s hard drives is predicted to quickly reach its limits at an areal density of about a terabyte per square inch. When this density is reached, the magnetic grains that define the magnetic state of the bits will become thermally unstable. This is known as the superparamagnetic limit in physics. The use of new alloys with better thermal stability is proposed to decrease the grain sizes without compromising the data fidelity. The write fields required for these alloys are prohibitively large, however. Fortunately, the write field decreases with increasing temperature. To this end, heat-assisted magnetic recording (HAMR) has emerged as a promising approach. In HAMR, a laser is integrated to the magnetic read/write head to heat the storage medium locally to decrease the write fields near the bit to be written. Major hard drive companies such as Seagate and Hitachi are already considering this technique for their next-generation magnetic disks.

A group of researchers from Korea and the United States take a step closer to the realization of HAMR and tackle several issues toward this end. One of the challenges for HAMR is the integration of optics with the magnetic head on a small footprint. Cho et al. introduced a polymeric light waveguide delivery system integrated on a conventional magnetic head. This new light delivery design offers advantages such as low cost, easy fabrication, and process compatibility with the current magnetic technology. The polymer material used in the waveguide’s fabrication has the potential for inexpensive HAMR heads, which will be crucial for widespread availability in the consumer market. The low process temperature for the polymer waveguides makes it also compatible with the current magnetic head fabrication. Another advantage of the design by Cho et al. is its use of fiber-based waveguide couplers with a small footprint. Currently proposed HAMR devices are based on free-space optics such as lenses, making the magnetic head too bulky for real hard drive applications.

The biggest requirement for the realization of HAMR is still the availability of an optical spot on the nanometer scale, which needs to be comparable to the size of the magnetic bits. The smallest optical spot available with conventional diffraction-limited optics, e.g, with a lens, is still orders of magnitude larger than the magnetic grains. This makes plasmonics, or metal-optics, instrumental for the future of HAMR. Plasmonics that uses metals for optics has the potential to enhance and confine light on a nanoscale through surface plasmons in metals. All the HAMR approaches proposed to date rely on plasmonic designs to achieve a tiny laser spot to heat the magnetic medium locally. The design by Cho et al. is no exception; it uses a C-shaped aperture in a metal surface to achieve a 100-nm optical spot.

Even though HAMR is emerging as a potential technology, real deployment of HAMR in commercial hard drives is still many years ahead. At this point, the proof-of-concept devices demonstrated cannot offer areal densities higher than those available with the current magnetic hard disk drives. Many practical aspects of HAMR such as thermal management, e.g. what happens when the heat is elevated to 300K locally, need to be addressed first. Let us watch closely whether we will indeed have any lasers in our hard drives in the future.